1. Field of the Invention
This invention relates to optical networks, and more particularly, to optical wavelength routing mechanisms.
2. Description of Related Art
In telecommunications, optical fiber has become one of the most successful transmission media due to its high transmission rates and low error rates. Driven by subscriber demand, network owners are currently deploying systems capable of supporting fiber for business and residential applications.
In known systems, the synchronous optical network (SONET) standard defines the physical interface and optical line rates known as optical carriers (OC) signals, a frame format and an OAandM protocol. User signals are converted into a standard electrical format called the synchronous transport signal (STS), which is the equivalent to the format of the optical signal (OC).
As the bandwidth and infrastructure needs increase, fiber exhaust is becoming rapidly growing problem. A solution to fiber exhaust is to use a plurality of channels on the same fiber, the channels being routed separately according to their wavelength, a technique termed wavelength division multiplexing (WDM).
WDM transmission can provide manifold capacity expansion on existing fiber links. Its potential for routing signals is equally important. By introducing WDM, the capacity of a ring can be increased in an efficient and cost-effective way with a 100% multiplex section protection, and with minimal changes to the nodes hardware or to the automatic switching protocol (ASP).
Single and multihop WDM network architectures have been studied. See, for example, xe2x80x9cDense Wavelength Division Multiplexing Networksxe2x80x9d by C. A Brackett, IEEE Journal on Selected Areas in Communications, Vol. 8, No. 6, 1990, pp. 948-964; and xe2x80x9cTerabit Lightwave Networks: Multihop Approachxe2x80x9d by A. S. Acampora et al., ATandT Technical Journal, November-December 1987, pp. 21-34.
Further discussions on local area networks are found in the following, xe2x80x9cDense Wavelength Division Multiplexing Networksxe2x80x9d, C. A. Brackett, IEEE Journal on Selected Areas in Communications, vol. 8, No. 6, 1990, pp. 948-964, xe2x80x9cTerabit Lightwave Networks: Multihop Approachxe2x80x9d, A. S. Acampora et al., ATandT Technical Journal, November-December 1987, pp. 21-34, The Lambdanet Multiwavelength Network: Architecture, Applications and Demonstrations, M. Goodman et al., IEEE Journal on Selected Areas in Communications, vol. 8, No. 6, 1990, pp. 995-1004 and xe2x80x9cPerformance Analysis of Multihop Lightwave Networks with Hot Potato Routing and Distance-Age Prioritiesxe2x80x9d, Z. Zhang et al., IEEE Transactions on Communications, vol. 42, No. 8, August 1994, pp. 2571-2581.
A typical network architecture in the metro area can be subdivided into both an access and a core network, each with a unique set of characteristics. The core portion of current metropolitan networks face serious challenges adapting to the emergence of high bandwidth IP services. Current core deployments are based almost exclusively on SONET ring architectures, most often BLSR""s. In the past, these rings have provided a cost-effective means to transport core traffic which is mainly central office (CO) to CO circuit-switched demand.
While these networks can efficiently manipulate low-bandwidths voice-oriented traffic, they become significantly less effective when faced with high-capacity IP services. Today, such demand is often routed over SONET rings simply to provide the protection switching speeds that have become an essential element of many service level agreements. In areas where fiber exhaust is a concern, a problem now emerging in the metropolitan environment, these multiple overlaid SONET rings can be aggregated over a single set of fibers through the use of dense wavelength division multiplexing (DWDM) systems. Since most of the systems commercially available today are configured as point-to-point DWDM terminals, they must be placed xe2x80x9cback-to-backxe2x80x9d for use in a SONET architecture. This complex configuration can result in considerable capital equipment outlay and the operational complications associated with multiple equipment layers.
At each node in the network, some wavelengths may be dropped or added into the data stream. In situations where video or other audio media are being xe2x80x9cbroadcast,xe2x80x9d there may be a need for drop-and-continue functionality of the nodes. This allows the nodes to drop data for users receiving the broadcast but also continuing that information on to other nodes where other users may also be tuned into the same broadcast.
Optical network designers have long desired to find simpler and cheaper equipment for performing add/drop function at each node. Particularly in WDM systems, designers have desired to use tunable filters to dynamically remove wavelengths or lambdas (xcex""s) from incoming light stream. There are significant cost and design advantages to using tunable filters that can dynamically scan and drop lambdas across a wide spectrum of wavelengths.
Unfortunately, one limitation of using tunable filters in optical wavelength routing relates to the undesirable and inadvertent dropping of wavelengths and signal in a WDM or DWDM network when the tunable filter scans from wavelength to nonadjacent wavelength. This is data leakage. For example, scanning from wavelength 1 to say wavelength 5, the tunable filter would end up dropping each wavelength in between (2, 3, and 4) that the filter traverse to reach wavelength 5. From a quality of service perspective, the dropping of these wavelengths would not be acceptable as these other wavelengths in a multi-wavelength WDM system are also carrying data required by the current or other nodes. The dropping of wavelengths could cause data to be delayed or more likely lost while the tunable filter traverses its passband from its current wavelength to the desired wavelength to be dropped. Additionally, some known systems may be able to scan a few frequencies but they cannot typically scan the entire spectrum of signal wavelength without adding a significant number of parts at increased bulk and cost.
Accordingly, there is a need for improved equipment designs that address the dropped wavelength situation associated with tunable filters. The use of tunable filters could greatly reduce the number and types of switches and other parts used in equipment architecture for DWDM systems. A further need exists for tunable devices that traverse the majority of the wavelengths used in the data spectrum to provide dynamic optical wavelength drop/continue. It is further desired that the device be of sufficient speed so as to minimize any amount of data that may be lost or delayed during tuning.
The present invention provides devices and methods for improving optical network equipment architecture. More specifically, the present invention addresses the undesired data leakage or signal drop when tunable filters attempt to move their passband from one signal wavelength to a nonadjacent wavelength. Although not limited to the following, the present invention is of particular use with Fabry-Perot type tunable filters.
In one aspect of the present invention, an optical assembly having a light circulating device is used for routing light and may be used with a receiver. The assembly includes a tunable filter for receiving light traveling from the light circulating device and for selecting which wavelengths of light that reach the receiver. A wavelength routing mechanism optically coupled to the tunable filter and located downstream from the circulator is used to prevent undesired dropping of signal wavelengths. The mechanism has a first operational mode preventing light from reaching the receiver and a second operational mode allowing light to reach the receiver. When the filter is tuning, the mechanism is preferably in the first operational mode. The receiver may be a device that receives signals in the optical domain and converts them into the electrical domain.
In another aspect of the present invention, an optical assembly having a light circulating device and a tunable filter is used to deliver signals to a receiver. The tunable filter receives light traveling from the input of the light circulating device and has a passband that allows light at a first signal wavelength to pass while reflecting other signal wavelengths back to the light circulating device along a through path from the filter to the output of the light circulating device. A wavelength routing mechanism optically coupled to the tunable filter is adapted to direct light at wavelength (or wavelengths in some situations) of the passband along at least a portion of the through pathway back to the light circulating device. Preferably, the tunable filter has a normal operating mode and a tuning mode, whereby light only reaches the receiver when the tunable filter is in normal operating mode. More preferably, the assembly an arrangement of mirror reflections on the first path and the second path so that an even number of mirror reflection are experienced on one path and an odd number on the other path as light is recombined at the coupler.
In another aspect of the present invention, an optical assembly for for directing light and for use with a receiver comprises a light circulating device having at least an input and an output. A tunable filter receives light traveling from the light circulating device. The assembly further includes a wavelength routing mechanism optically coupled to the tunable filter. The mechanism has an operational mode preventing light from reaching the receiver by directing wavelengths passing through the tunable filter along a path extending outwardly from the wavelength routing mechanism towards a light combiner for joining light on the first path with light on a second path extending outwardly from the output of the light circulating device. The assembly preferably includes a polarization rotating device disposed along a first pathway for changing polarization of light on the first pathway prior to joining light from said second pathway at said coupler.
These and other embodiments of the present invention, as well as its advantages and features, are described in more detail in conjunction with the text below and attached figures.